Over the past decade, numerous advances have been made in the synthetic procedures for formation and isolation of high quality inorganic nanoparticles. The size dependent optical properties of colloidal semiconductor nanocrystals are ideal for applications in fields ranging from biological imaging, photovoltaics optoelectronic devices, biological tagging, optical switching, solid-state lighting, and solar cell applications. (Chan, W.; Nie, S. Science. 1998, 281, 2016-2018; Bruchez, J.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. Science. 1998, 281, 2013-2016; Bharali, D. J.; Lucey, D. W.; Jayakumar, H.; Pudavar, H. E.; Prasad, P. N. Journal of American Chemistry Society. 2005, 127, 11364-113671; Huynh, W.; Dittmer, J.; Alivisatos, A. Science. 2002, 295, 2425-2427; Sun, B.; Marx, E.; Greenham, N. Nano Letters. 2003, 3, 961-963; Gur, I.; Fromer, N.; Chen, C.; Kanaras, A.; Alivisatos, A. Nano Letters. 2007, 7, 409-414). Due to the ease of synthesis, a significant fraction of the research to date has centered on the II-VI family of semiconductors utilizing the type I core-shell structures, namely CdSe/ZnS (Park, J.; Joo, J.; Kwon, S.; Jang, Y.; Hyeon, T. Angew. Chem. Int. Ed. 2007, 46, 4630-4660; Embden, J.; Jasieniak, J.; Gómez, D.; Mulvaney, P.; Giersig, M. Aust. J. Chem. 2007, 60, 457-471). Recent interest has focused on developing routes to the III-V family, namely InP (Xie, R.; Battaglia, D.; Peng, X. Journal of American Chemistry Society. 2007, 129, 15432-15433; Gerbec, J.; Magana, D.; Washington, A.; Strouse, G. Journal of American Chemistry Society. 2005, 127, 15791; Adam, S.; Talapin, D.; Borchert, H.; Lobo, A.; McGinley, C.; de Castro, A.; Hasse, M.; Weller, H.; Möller, T. The Journal of Chemical Physics. 2005, 123, 084706; Borchert, H.; Haubold, S.; Haase, M.; Weller, H.; McGinley, C.; Riedler, M.; Moller, T. Nano Letters. 2002, 2, 151-154), due to the perceived lower toxicity for InP based nanocrystals (Bharali, D. J.; et al., Journal of American Chemistry Society. 2005, 127, 11364-113671; Xie, R.; et al., Journal of American Chemistry Society. 2007, 129, 15432-15433; Oda, K. Industrial Health. 1997, 35, 61-68; Zheng, W.; Winter, S. M.; Kattnig, M. J.; Carter, D. E.; Sipes, I. G. J Toxicol Environ Health. 1994, 43, 483-494; Kabe, I.; Kazuyuki, O.; Hiroshi, N.; Nomiyama, T.; Uemura, T.; Hosoda, K.; Ishizuka, C.; Yamazaki, K.; Sakurai, H. Journal of Occupational Health. 1996, 38, 6-12; Yamazk, I.; Tanaka, A.; Hirata, M.; Omura, M.; Makita, Y.; Inoue, N.; Sugio, K.; Sugimachi, K., Journal of Occupational Health. 2000, 42, 169-178). The downside to InP is the poor photoluminescence (PL) quantum yield (QY), which is typically <4% once isolated from the reaction mixture, although core-shelling yields a value of ˜20% depending on size (Borchert, H.; et al., Nano Letters. 2002, 2, 151-154; Haubold, S.; Haase, M.; Kornowski, A.; Weller, H. Chem Phys Chem. 2001, 2, 331-334). The poor PL QY performance for these materials can be traced to the presence of phosphorus vacancies (Adam, S.; et al., The Journal of Chemical Physics. 2005, 123, 084706) (VP) in the material.
The general synthetic approach for preparation of colloidal semiconductor nanoparticles employs a bulky reaction flask under continuous Ar flow with a heating mantle operating in excess of 240° C. The reaction is initiated by rapid injection of the precursors, which are the source materials for the nanoparticles, at high temperatures and growth is controlled by the addition of a strongly coordinating ligand to control kinetics. And to a more limited extent, domestic microwave ovens have been used to synthesize nanoparticles. The high temperature method imposes a limiting factor for industrial scalabiity and rapid nanomaterial discovery for several reasons: random batch-to-batch irregularities such as temperature ramping rates and thermal instability; time and cost required for preparation for each individual reaction; and low product yield for device applications.
While recent advances in the field have developed better reactants, including inorganic single source precursors, metal salts, and oxides; better passivants, such as amines and non-coordinating solvents; and better reaction technologies, such as thermal flow reactors; the reactions are still limited by reproducibility. Coupled to this problem is the lack of control over reaction times, which require continuous monitoring. In the case of III-V compound semiconductors, the synthetic pathways have rates of growth on the order of days, while in the case of II-VI's, size control is very difficult and depends on the ability to rapidly cool the reaction. In these cases, the reaction depends on heating rate, heat uniformity over the reaction vessel, stirring and rapid and uniform cool-down.
Removal of the surface VP sites by active ion etching with hydrofluoric acid (HF) improves the PL performance of these materials to ˜40% (Gerbec, J.; et al., Journal of American Chemistry Society. 2005, 127, 15791; Adam, S.; et al., The Journal of Chemical Physics. 2005, 123, 084706; Micic, O.; Sprague, J.; Lu, Z. Nozik, A. Appl. Phys. Letters. 1996, 68, 3150-3152; Talapin, D.; Gaponik, N.; Borchert, H.; Rogach, A.; Haase, M.; Weller, H. Journal of Physical Chemistry B. 2002, 106, 12659-12663). While the use of active ion etching with HF enhances the InP nanocrystal PL, it represents an inconvenient extra synthetic step that lowers solubility, broadens the excitonic absorption line width, and increases the difficulty for ZnS shelling. The development of an in-situ active ion etchant can simplify the preparation of this family of material, improve the PL QY, and maintain the optical properties of the nanocrystal.
The use of ionic liquids in synthesis have attracted attention due to the high thermal stability of the solvent, non-reactivity of the materials, and the added benefit of the solvent being recyclable (Antonietti, M.; Kuang, D.; Smarsly, B.; Zhou, Y. Angew. Chem. Int. Ed. 2004, 43, 4988-4992). In an earlier report, our group demonstrated the advantages of using non-fluorinated ionic liquids in MW chemistry to accelerate growth of InP and CdSe nanocrystals (Gerbec, J.; et al., Journal of American Chemistry Society. 2005, 127, 15791). The isolated InP showed typical PL QYs for this family with values on the order of 4%. Following HF treatment the PL QY increases to 38% (Id.). The most notable effect of using an ionic liquid in MW chemistry is the efficient conversion of MW energy into thermal energy due to the high MW cross-section that ionic liquids possess. In MW chemistry, the molecule with the highest cross-section selectively absorbs the MW energy and through relaxation heats the solvent or the molecular precursors. The selective absorption leads to the “specific” microwave effects often quoted in the synthetic literature (Kappe, O. Angew. Chem. Int. Ed. 2004, 43, 6250-6284). Ionic liquids typically are not directly involved in the reaction mechanism and can be considered a spectator solvent, allowing non-absorbing materials to be rapidly heated in the MW by convective loss; although ILs are believed to enhance reactions due to the highly ordered solvent framework (Antonietti, M.; et al., Angew. Chem. Int. Ed. 2004, 43, 4988-4992; Redel, E.; Thomann, R.; Janiak, C. Chem. Commun. 2008, 1789-1791).
Nanocrystals, as well as the other systems, are rapidly finding applications in biological imaging, biomedical technologies, electronics and photovoltaics. However, current methods to remove materials defects, vacancy or add-ion removal requires annealing (long reaction time) or post reaction treatment with a highly dangerous (HF) solvent. The invention alleviated this shortcomings, allowing rapid isolation of materials and improved total yields.